1. Field of the Invention
The present invention relates to an optical pickup device configured to read information recorded on optical discs or to write information onto optical discs.
2. Description of the Related Art
Optical pickup devices for reading information recorded on optical discs or writing information onto optical discs have been known (for example, see Japanese Patent Application Laid-Open Publication No. 2011-40123). Optical pickup devices are equipped with light sources, objective lenses, sensor lenses, and light-receiving units. Laser light emitted from the light source is converged by the objective lens onto the recording plane of the optical disc. Return light reflected at the recording plane of the optical disc is given an astigmatism by the sensor lens and then received by the light-receiving unit. The light-receiving unit generates a focus error signal based on the return light it receives. Note that the focus error signal is a control signal used for focus servo control in order to match the focal position of the laser light from the objective lens to the recording plane of the optical disc.
In conventional optical pickup devices, the lens surface of the sensor lens is constituted by a cylindrical surface. Return light reflected at the recording plane of the optical disc is incident on the lens surface of the sensor lens.
In recent years, as equipment that incorporates optical pickup devices has become more compact, the need has arisen for smaller optical pickup devices. In order to make optical pickup devices smaller, distances between various components of the optical pickup device must be reduced. When the distance between the sensor lens and the light-receiving unit is reduced, the curvature radius of the lens surface of the sensor lens must be reduced in order to generate the desired focus error signal. For example, when the distance between the sensor lens and the light-receiving unit is 3.0 mm or less, the curvature radius of the lens surface of the sensor lens must be 3.0 mm or less. However, when the curvature radius of the lens surface of the sensor lens is reduced, the sensor lens is more prone to assembly discrepancies, which therefore creates the problem of the performance of optical pickup devices becoming unstable.
Note that methods are also conceivable in which the distance between the sensor lens and the light-receiving unit is reduced without changing the curvature radius of the lens surface of the sensor lens. In such cases, optical components of glass or the like must be disposed between the sensor lens and the light-receiving unit in order to increase the optical distance between the sensor lens and the light-receiving unit. However, by increasing the optical distance between the sensor lens and the light-receiving unit, the size of the spot of return light on the light-receiving unit is reduced, thus creating the problem of greater sensitivity to positional shift of the spot of return light.